Chip Detector Caution After Compressor Wash on an Arriel 1D1
An AS350 B2 with an Arriel 1D1 has 4,120 hours since new and 1,870 hours since the last hot-section inspection. After a compressor wash, during the subsequent ground run, the chip detector caution illuminates for about three seconds and then extinguishes. The magnetic plug has fine ferrous fuzz but no flakes. Oil analysis shows iron at 18 ppm against a previous 9 ppm; aluminium is unchanged at 4 ppm. The oil filter is cut open and contains no metallic particles. Borescope of the compressor shows no obvious damage. The operator wants to keep the aircraft on a five-day survey and asks you to 'clean the plug and sign it out'. The maintenance manual requires action after a chip warning and gives a limit for fuzz versus particles.
1. Interpret the chip detector and oil analysis evidence. Is the engine airworthy for continued service as requested?
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A transient chip caution with fine fuzz and no flakes is less alarming than a steady warning with visible particles, but it is not automatically benign. The rise in iron from 9 to 18 ppm is a trend change, not just a one-off. The absence of filter debris and the clean borescope reduce the likelihood of a large failure, but they do not prove the engine is healthy; early bearing or gear wear can produce fine fuzz before visible particles appear.
The manual's fuzz-versus-particle limit is the controlling document. If the deposit is within the fuzz limit, the engine may be eligible for cleaning, a ground run and recheck. If it exceeds the limit, or if the caution repeats, the engine should be removed for investigation. I would not sign it out solely on the operator's request. The correct decision is conditional: clean, inspect, run, and recheck against the manual's criteria, with a clear plan if the chip caution returns.
2. What maintenance actions and ground runs would you carry out before further flight?
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I would record the chip event and the oil analysis results. I would remove and clean the magnetic plug, inspect the removed fuzz under magnification, and check for flakes or gritty material. I would cut and inspect the oil filter element again, check the oil screen if fitted, and verify oil type and quantity. I would perform the manual's chip-detector functional check.
Then I would run the engine on the ground at idle and moderate power, monitoring chip caution, oil pressure, oil temperature and any unusual noise. After shutdown I would re-inspect the plug and chip detector. If the plug stays clean and the caution does not recur, the engine may be released only if the manual permits continued service and the oil trend is acceptable. I would also repeat oil analysis at a shorter interval, for example after 10 to 15 hours, and review the trend before the aircraft resumes normal survey flying.
3. If the chip caution recurs after cleaning, what decision and documentation would be required?
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A recurring caution after cleaning is a strong indication that material is still being generated. At that point I would not continue cleaning and releasing. I would ground the engine, raise a defect, and recommend removal for inspection or overhaul as required by the engine manual and the operator's approved maintenance programme. The decision should be based on the recurrence, the plug condition, oil analysis trend and any performance change, not on the aircraft's schedule.
Documentation would include the initial event, cleaning action, ground-run results, recurrence, oil analysis values, filter inspection and the removal recommendation. The maintenance release for further flight would not be issued. If the operator wants to ferry the aircraft, that would require a specific ferry approval and engineering assessment, not a normal release. I would also notify the maintenance controller and record the delay in the aircraft log so the decision trail is clear.